1. Field of the Invention
The present invention relates to a process for purifying a crude 1,4-naphthoquinone (hereinafter referring to as naphthoquinone) obtained by an oxidation of naphthalene.
2. Description of the Prior Arts
Heretofore, naphthoquinone especially 1,4-naphthoquinone has been produced by an oxidation of naphthalene, for example, by a process for oxidizing naphthalene in liquid phase with an oxidizing agent such as hydrogen peroxide in the presence of boric acid or a process for oxidizing naphthalene by a catalytic vapor phase oxidization. However, large amount of impurities such as acidic impurities and polycondensed impurities have been produced together with naphthoquinone in these processes whereby a highly purified naphthoquinone could not be easily obtained.
In the conventional catalytic vapor phase oxidation of naphthalene, a reaction mixture gas (naphthoquinone, phthalic anhydride, maleic anhydride, and naphthalene) produced by an oxidation of naphthalene, is trated with water to collect naphthoquinone and impurities of phthalic anhydride and maleic anhydride, with or without condensing phthalic anhydride in a pretreatment, and then, naphthoquinone is separated from water soluble impurities of phthalic acid and maleic acid, etc. to obtain naphthoquinone in an industrial process.
However, the resulting crude naphthoquinone usually contains impurities of phthalic acid, benzoic acid, maleic acid and polycondensed naphthoquinone materials derived from naphthoquinone.
A highly purified naphthoquinone has been obtained by the methods such as sublimation method, solvent extraction method and recrystallization method. However, the solvent extraction method and the recrystallization method suffer such disadvantages that highly purified naphthoquinone is not easily obtained by recrystallization and use of large volume of an organic solvent as a recrystallizing solvent is particularly dangerous, because of its firing and toxicity in the industry and the yield is usually very poor. On the other hand, the sublimation method has been industrially employed however, a thermal polycondensation of a crude naphthoquinone is easily casued and accordingly, the sublimation method should be carried out in a fluidized bed process carrying naphthoquinone with a large amount of air etc. below the melting temperature of naphthoquinone (Japanese Patent Publication No. 23013/1965 and W. Ger. Pat. No. 1,232,943). However, these conventional sublimation method suffers such disadvantages that (1) efficiency of an equipment is low and heat loss is large, because of its low vapor density, (2) the possibility of firing and explosion is present because of using large volume of air, which is very cheap, and (3) it is very difficult to obtain a highly purified naphthoquinone only by this method.
On the contrary, a distillation method has many disadvantages in an industrial process that (1) the carrier gas such as air does not need to be used because of its high vapor density and (2) a compact plant is possible to be used because of its high heat transfer. Nevertheless, the distillation method has not been used to get highly purified naphthoquinone because a polycondensation reaction of naphthoquinone easily occurs to get a pitch at the temperature higher than its melting point.